1. Field of the Invention
This invention relates to a gas turbine engine and more particularly to air separator and sealing structure in the turbine portion thereof providing a thrust balance piston to counteract the thrust of the turbine rotor.
2. Description of the Prior Art
The present trend in high efficiency combustion turbine engine design is to increase their firing temperature and effective pressure ratio. As a result, the engine net thrust also increases. The conventional manner of accommodating increased thrust is to either use a larger thrust bearing capable of withstanding the larger thrust but which is more expensive and also increases the heat loss, or to create a "balance piston" comprising high pressure air acting on the rotor in opposition to the rotor thrust, thus reducing bearing load.
In such a balance piston, high pressure air is introduced to the pressure face of the structure acting as the piston. The air pressure acts on the effective area of the pressure face to counteract the engine thrust. Use of this high pressure air for other than generating power in the turbine reduces the overall engine efficiency and thus both methods generally increase the expense of the engine and reduce its performance.
Presently, the assignee of the present invention manufactures a gas turbine engine having certain structure in the compressor section thereof which is subjected to high pressure air from the compressor to provide a thrust balance piston in opposition to the net rotor thrust. However, introduction of high pressure air into this relatively low pressure area of the compressor, to obtain a sufficient differential in pressure on the opposite faces of the balance piston structure to generate the thrust, is generally inefficient in that it also permits a relatively large amount of leakage of the high pressure air. This lost air is returned to the compressor for re-compression without providing any useful work, and, in at least the particular gas turbine engine above referred to, the effective area of the pressure face is relatively small.
U.S. Pat. No. 2,966,296 shows a gas turbine engine with a thrust balancing structure in the turbine section. The description therein is sufficient to generally describe a gas turbine and the function and operation of the thrust balancing feature and to describe in detail the particular structure appropriate for the particular gas turbine shown. The thrust balancing air in this patent is routed to individual downstream turbine stages, requiring many labyrinth seals and in fact, is routed to certain downstream chambers wherein the air pressure opposes the thrust balancing force on the opposite face of each rotor disc stage. Further, in that it is necessary to obtain flow throughout the path of this pressurized air to the downstream chambers, the various seals must be maintained within relatively close tolerances to permit such flow and yet provide the desired pressure drop to produce the balancing thrust.